libevent: Import libevent 2.1.12

MFC after: 2 weeks

Merge commit '5223d1d95fddcef6f9a36e264a5800bd907ade8b' into main

Copy libevent sources to contrib

To replace the libevent embedded in ntp, for OpenSSL 1.1.1 compat.

Approved by: re (gjb)